THE STEREOSCOPE : ITS HISTORY. 45 



These experiments are delightfully surprising when successfully- 

 accomplished for the first time. They are well worth the trifling pre- 

 liminary trouble which they entail. But even this can be in great 

 measure avoided by having a photograph of the picture taken on glass. 

 If you will previously approach the polite photographer in your most 

 charmingly courteous and irresistible style, enable him to perceive the 

 glittering phantom cone reversed in mid-air, invite him to grasp it 

 and give to this " airy nothing a local habitation and a name," and 

 convince him that, if not illusive, it is even more elusive than the 

 merry sunbeam which his camera alone can catch in all its beauty, he 

 will at once be lost in admiration of your magic skill and singular sa- 

 gacity, and instantly find it impossible to avoid preparing the wonder- 

 working photograph on glass. This he will smilingly present to you 

 in the most enthusiastic and complimentary manner, with evident 

 gratitude for the favor you have bestowed, and the good taste you 

 have exhibited in selecting him as the recipient of your discriminating 

 and exclusive confidence. 



The presence of the uncombined images at the sides of the bi- 

 nocular picture, as it stands out in solid relief, is apt to be confusing, 

 because their effect is partially to distract the attention. In Wheat- 

 stone's first experiments, he avoided them by looking through tubes, 

 or into a box. In any case, the methods of stereoscopy just de- 

 scribed, although by far the most useful in studying the principles of 

 binocular vision, are not usually acquired until after a few trials. 

 When they are once mastered, it becomes easy to discard pencils and 

 other points of fixation, and the voluntary muscular control of the 

 eyes is sufficient for all cases. Wheatstone gave to the world a new 

 revelation in both the science and the art of perspective, when, in 

 1838, he devised his reflecting stereoscope for the purpose of remov- 

 ing the difficulties involved in stereoscopy by direct vision. Figs. 2 

 and 3 are exact reproductions of his drawings, representing the front 

 view and ground-plan of his original stereoscope ; and, in describing 

 them, we can not do better than again to give his own words : "AA' 

 are two plane mirrors, about four inches square, inserted in frames, 

 and so adjusted that their backs form an angle of 90 with each other; 

 these mirrors are fixed by their common edge against an upright, B, 

 or against the middle line of a vertical board, cut away in such manner 

 as to allow the eyes to be placed before the two mirrors. C C are two 

 sliding boards, to which are attached the upright boards DD', which 

 may thus be removed to different distances from the mirrors. To 

 facilitate this adjustment I employ a right- and a left-handed wooden 

 screw, r I ; the two ends of this compound-screw pass through the 

 nuts e e', which are fixed to the lower parts of the upright boards D D', 

 so that, by turning the screw-pin p one way, the two boards will ap- 

 proach, and, by turning it the other way, they will recede from each 

 other ; one always preserving the same distance as the other from the 



